Organic electroluminescent devices have advantages that they do not require a backlight because they emit light on their own, differently from a liquid crystal display (LCD), and they can be manufactured to have a thickness of less than several nanometers to be used as thin and light display devices.
The organic electroluminescent devices are also drawing attention as next-generation display devices because the structure and manufacturing process of the organic electroluminescent devices are relatively simple and the production cost is low.
Particularly, since the organic electroluminescent devices have high luminous efficiency and low driving voltage, they consume a small amount of power. Since they have a fast pixel response rate, they can present high-quality video. When the organic electroluminescent devices are applied to color display devices, colors are realized with excellent quality and the color display devices are light, thin, and small, which results in high-quality display elements of portable information communication devices.
Colors are realized in the organic electroluminescent devices with the following methods: a side-by-side deposition method, a color change media (CCM) method, and a color filtering method. The side-by-side deposition sequentially deposits red (R), green (G), and blue (B) electroluminescent layers by using a delicately patterned metal mask based on minute transfer of the mask. The CCM method realizes green and red colors based on energy down-conversion of blue color. The color filtering method realizes colors by using a color filter and a white light emitting diode.
Among them, the color filtering method uses a shadow mask of which the entire light emitting area is exposed, instead of the delicately patterned metal mask required in the side-by-side deposition method. Thus, the color filtering method is an efficient manufacturing method and it yields a low defect rate. Therefore, the color filtering method can reduce the production cost.
Due to the advantages with respect to manufacturing process and cost, researchers are actively studying a white light emitting organic electroluminescent device using a color filter for a full-color display. The organic white light is also drawing attention as an environment-friendly light source, because it can replace the conventionally used gas-filled lamps using heavy metal elements or environmentally hazardous gas, such as mercury lamps and fluorescent lamps.
According to the conventional technology for realizing white light emitting organic electroluminescent devices, there are an organic electroluminescent device using colors in complementary relationships such as deep blue and yellow, and sky blue and red, which is shown in FIG. 1, and an organic electroluminescent device realizing white light by appropriately mixing three primary colors, i.e., blue, green, and red, which is shown in FIG. 2.
In short, the conventional white light emitting organic electroluminescent devices shown in FIGS. 1 and 2 acquire the white light by simply and sequentially disposing anodes A01 and B01 on substrates A10 and B10, disposing hole injection layers (HIL) A05 and B05, hole transport layers (HTL) A06 and B06, and a plurality of electroluminescent layers A07, A09, B07, B09, and B11, each including red, green, or blue light emitting substance, electron transport layers (ETL) A08 and B08, electron injection layers (EIL) A15 and B15, and cathodes A04 and B04.
The organic electroluminescent devices of FIGS. 1 and 2 emit light in the following procedure. When a voltage is applied to the space between the anode A01 or B01 and the cathode A04 or B04, holes injected from the anode pass through the hole transport layer A06 or B06 to the electroluminescent layers A07, A09, B07, B09, and B11. Electrons are injected from the cathode A04 or B04 through the electron transport layer A08 or B08 to the electroluminescent layers. Carriers are recombined in the interface and the bulk of the hole transport layer and the electroluminescent layers to thereby produce excitons.
The generated excitons are distributed to the respective electroluminescent layers and shift into the ground state. Thus, blue, green, and red colors are realized according to the colors of the electroluminescent layers and white light is emitted.
FIG. 3 shows a tandem organic electroluminescent device having a complicated multiple layer structure to increase efficiency according to the related art. When a voltage is applied, the organic electroluminescent device of FIG. 3 operates such that the number of electrons and holes injected to the anode C01 and the cathode C04 doubles by forming a p-type doped layer C20 and an n-type doped layer C21 out of the charge generation layer for additionally generating electrons and holes though a complicated doping process, or by forming a transparent anode, e.g., indium tin oxide (ITO), in the organic layer by using a sputtering method.
In short, the organic electroluminescent device of FIG. 3 includes a first element and a second element disposed in the anode on the substrate C10. The first element includes the hole injection layer C05, the hole transport layer C06a, the electroluminescent layer C07a, the electron transport layer C08a, and the n-type doped layer C20. The second element includes the p-type doped layer C21 or a transparent anode formed by using a sputtering method, the hole transport layer C06b, the electroluminescent layer C07b, the electron transport layer C08b, the electron injection layer C15, and the cathode C04.
Among the aforementioned conventional methods, the organic electroluminescent device of FIG. 1 that utilizes the complementary color relationship or the device of FIG. 2 having a simple stack structure where three primary color electroluminescent layers are stacked necessarily require two to three electroluminescent layers to realize white light in the organic layer used for injection and transfer of the electrons and holes. The organic electroluminescent device of FIG. 4 should properly distribute excitons generated in the three electroluminescent layers in the respect of a light emission mechanism.
In general, the organic electroluminescent device emits light and Joule heat caused by the emitted light, when voltage is applied thereto. The generation of Joule heat degrades the organic electroluminescent device and deteriorates its lifespan. A stack-type white device where electroluminescent layers of various colors are sequentially disposed undergoes remarkable lifespan deterioration as the number of electroluminescent layers increases.
The excitons produced by coupling between the holes and the electrons that are injected to the anode and the cathode when voltage is applied to the organic electroluminescent device exist in different areas according to the intensity of the applied voltage and current. This signifies that the light emission region is changed. The change in the light emission region becomes more distinctive when the number of electroluminescent layers is large. In the case of the conventional white light emitting device having a stack structure, the light emission spectrum of each color of RGB is changed as the intensity of the voltage or current becomes different. Therefore, the entire light emission color is changed.
The organic electroluminescent device is fabricated by performing doping onto the electroluminescent layers to increase the luminous efficiency. The doping process is a process for forming a layer by mixing a host material and a guest material. In the case where the layer is fluorescent, the portion generally occupied by the guest material reaches 1 to 10% of the entire materials. In the case where the layer is phosphorescent, it reaches 5 to 20% of the entire materials. With respect to the manufacturing process, doping is one of the most difficult processes, because an organic layer should be formed while the ratio of the small amount of the guest material is maintained. The presence of a different material in the electroluminescent layer changes the light emitting characteristic and reduces property reproducibility of the organic electroluminescent device. In the tandem white light emitting device shown in FIG. 3, the doping process is quite complicated, because doping should be performed onto the p-type doped layer (C20) and the n-type doped layer (C21) other than the electroluminescent layer, or sputtering for an anode should be performed onto an organic layer.